In a wireless communication system, an A interface provides an interface between a radio access network and a core network. A base station controller (BSC) communicates with mobile switching center (MSC) through an A1 interface or an A2 interface of the A interface. The A1 interface is a signaling interface used to transmit signaling information between the BSC and the MSC, and the A2 interface is the service interface used to transmit voice information between the BSC and the MSC.
IP transformation is a current trend in network development. The A interface over IP is defined in the Interoperability Specification (IOS) 5.0 of the Third Generation Partnership Project 2 (3GPP2).
FIG. 1 shows the reference model of the A interface over IP in a prior wireless communication system. As shown in FIG. 1, the MSC is divided into an MSC Emulation (MSCe) and a media gateway (MGW) based on the principle of separating bearer and control in the core network, and defines the A interface over IP as the A1p/A2p interface in IOS 5.0.
The MSCe fulfills the function of a control plane and the A1p interface is the interface between the BSC and the MSCe.
The MGW fulfills the function of a service plane, and the A2p interface is the interface between the BSC and the MGW.
FIG. 2 shows the signaling flow between a calling mobile station (MS) and a called MS during a call in the system shown in FIG. 1. The dashed line in FIG. 2 indicates the flow of service information, and the solid lines indicate the flow of control information. The flow as illustrated includes the following steps.
Step 201: The calling MS and called MS set up a call connection based on the flow specified by the IOS.
Step 202: The MGW plays a call progress indication to the calling MS.
Step 203: The called MS rings.
Step 204: The called MS answers the call and sends a response message to MSCe.
Step 205: The MSCe notifies the MGW to set up a voice path between the calling MS and the called MS after receiving the response message. The calling MS then communicates with the called MS.
In this step, because a transcoder (TC) is moved to the MGW from the BSC after the A interface is enabled over IP, the BSC encapsulates received voice data according, for example, to the Real-Time Transport Protocol (RTP), and sends the data to the MGW through the A2p interface. The MGW then sends the voice data processed by the TC to terminal through the corresponding BSC. If the calling MS and the called MS support the same voice coding and decoding types, the MGW can send the voice data directly to the BSC, as indicated by the arrow in FIG. 1. In this case, the TC need not process voice data between the called MS and the calling MS such as rate matching and code conversion. In this case, voice data is transparently transmitted in the MGW.
Analyzing, by the inventor of the application, the above method for transmitting voice data revealed that when the calling MS and the called MS support the same voice coding and decoding type, no processing of voice data is needed in the MGW, but the transparent transmission of the voice data in the MGW adds to the transmission delay of the voice data and wastes the processing capability of the MGW.